Metals are often coated to prevent corrosion. Corrosion of steel strips may be inhibited by applying an outer layer of zinc to the strips. The zinc layer is applied by a hot dip galvanizing or electrogalvanizing process, for example. After the zinc layer has been applied to the steel strip, the strip is subjected to a forming process such as stamping. In preparation for the forming process, the strip may be coated with a lubricant such as oil.
Lubricant may be applied prior to or during different stages of the forming process and may be applied by any number of techniques known in the art including electrostatically, by gravity and by "squeegee." One example of a lubricant that is applied to the steel is referred to as mill oil. Mill oil, which is primarily a rust preventative, is applied at the galvanizing facility to the uncoated galvanized steel prior to coiling. Another lubricant referred to as prelube may be applied by the galvanizing facility to the uncoated galvanized steel. Prelube provides both rust prevention and lubricity to the galvanized steel and is used when more difficult forming is to be carried out. At a stamping plant the steel may be subjected to blanking and the mill oil may be washed off by a blank washer. At the blank washer another lubricant may be applied to the uncoated product. Some stamping lines may stamp the product without applying any lubricant other than the mill oil. Any of the above lubricants may be present on the strip as the strip enters the stamping line and a drawing lubricant may be applied at a press.
During stamping, the steel strip is placed between an upper punch and a lower die of a press where it is held in place and formed into a desired shape. Articles such as automobile hoods and fenders may be formed in this manner. After forming, the article is passed through a bath containing an alkaline cleanser for removing the oil. The article may then be assembled, coated with a phosphate painting preparation and painted.
One factor that is important to the stamping process is the formability of the steel. A metal draw bead may be located adjacent to the die opening for retarding travel of the strip or blank as it is pushed into the die. The amount of frictional resistance at the interfaces between the punch, die and blank is an important factor in the quality of the formed article. Areas of both low and high strip-to-draw-bead speeds may occur during forming. It is desirable for the blank to exhibit a coefficient of friction with as little variation across low and high strip-to-draw bead speeds as possible. The high speed areas may result in a "hydroplaning" effect at a low frictional resistance whereas the low speed areas may suffer from slip stick. Slip stick is a phenomenon that occurs when two metals contact each other at slow relative speeds and high frictional resistance. In effect, a repeated cold welding and breaking-free of the metals occurs during slip stick. Notwithstanding proper die set up and punch operation, if there is excessive frictional resistance during forming, portions of the strip may tear or break, resulting in decreased quality or scrapping of the steel products.